The present disclosure relates generally to wind turbines, and more particularly to a load mitigation system for wind turbine blades.
Modern wind turbines typically comprise a plurality of flexible blades extending from a rotor hub. Three-bladed hubs are particularly common among large scale wind turbines. Wind incident on turbine blades rotates the hub with respect to a generator nacelle mounted atop a tower. This nacelle rides a bearing track or rail atop the tower, allowing the nacelle to be yawed so that the hub and blades face incoming wind. Similarly, each blade is mounted to the hub via a bearing assembly that provides the freedom of rotational movement necessary for each blade to be pitched to regulate aerodynamic load and energy capture from the wind. The blades of large wind turbines are usually pitched using hydraulic actuators, although non-hydraulic actuators are fairly common for smaller turbines. Some wind turbines are capable of separately pitching each blade to a different angle, while others pitch all blades identically. Blade pitching is used to avoid premature damage or wear to wind turbine components, and to control wind turbine generator speeds and torques. In particular, blades are commonly pitched to feather for aerodynamic unloading both during prolonged periods of dangerously high wind speeds, and during rapid gusts.
Several methods exist for controlling wind turbine generator speeds and power absorption by pitching wind turbine blades to feather in response to transient gusts. Some wind turbines sense rotor torque, and pitch blades for aerodynamically unloading in response to high torque events. Other wind turbines control blade pitch based on rotor speed, or based on strain sensed via strain gauges in each (or at least one) blade. All of these mechanisms experience a time lag between the onset of a heavy blade load (e.g. from a rapid gust) and subsequent pitch-unloading in response to the heavy blade load. Some wind turbines use twist-bend coupled flexible blades that passively twist to alleviate aerodynamic load as blades deflect under high wind speeds.